This project proposes enquiries into the ultrastructure of the atrium and, for comparative reasons, of the ventricles of mammalian hearts and the hearts of other vertebrate species. The atrium is chosen as the main topic of study because of its importance for impulse generation, and because of the interesting phenomena of impulse conduction encountered in the atrium. Since the atria, especially of mammals, are morphologically quite heterogenous it is important to define the anatomical details of the heterogeneity. New advances in contrasting procedures have made it possible to ask interesting questions regarding the geometry and quantity of structural features of cardiac cells. In this proposal we shall study such matters with conventional and high voltage electron microscopy, as well as with the freeze fracture method and scanning electron microscopy. Specifically we shall study the distribution of transverse tubules and sarcoplasmic reticulum in the common and specialized atrial cells, the distribution and configuration of the nexuses and the anatomy and distribution of the cytoskeleton and of the microfibrils at the surface of cardiac cells. Finally, we shall study with serial sections the geometry of cell appositions in bundles of atrial and ventricular fibers for modelling current distribution in such bundles.